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Food Microbiology The good, the bad and the ugly • Good-important in food production; provide better taste and texture • Bad-cause of food borne illness: – Infection with live organisms – Intoxication with bacterial toxins • Ugly-cause food spoilage with undesirable changes Factors affecting microbial growth in food • Intrinsic: conditions present in food – Water availability (aw): amount of water in food (pure water is 1.0) • Most food >0.98; most bacteria require >0.90 – pH: most pathogens not grow at pH<4.5 (except Lactic acid bacteria) – Nutrients • Extrinsic: – Storage temperature • <0⁰ no growth (water crystallizes) • Refrigerator: 4⁰C to 10⁰C (enzyme rxns very slow or non-existent – Atmosphere: availability of O2 1 Natural Food Protectants • Antimicrobial chemicals: – Egg whites-lysozyme – Cranberries-benzoic acid – Raw Milk- peroxidase system • Biological Barriers: rinds, shells, other coverings – Whole lemons last longer than slices – Some microbes can degrade Microorganisms in food and beverage production • Lactic acid fermentation: – GlucoseÆpyruvic acidÆlactic acid • Fermented milk products (cheese, yogurt) • Pickled vegetables (sauerkraut) • Alcoholic fermentation by yeast: – GlucoseÆpyruvic acidÆalcohol + CO2 • Wine, beer, distilled spirits • Yeast for bread production • Mold growth – Soy sauce/Bleu cheese Food Spoilage • Undesirable changes – Repugnant taste, odor, appearance: usually not harmful but pathogens may also be present • Common food spoilage microbes – Pseudomonas, Erwinia, Acetobacter, Alcaligenes, Lactobacillus, Leuconostoc – Fungus • Rhizopus, Penicillium • Aspergillus: grain and peanuts; produce aflatoxin • Claviceps purpures: grain; ergot poisoning 2 Food Preservation • Kill organisms: canning, pasteurization, cooking, irradiation • Inhibit growth: refrigeration, freeze, dehydration (reduce a w), lower pH, high salt or sugar, chemicals Food borne intoxication • Food borne intoxication: bacteria grow within the food and produce toxins, the toxins are what lead to food poisoning symptoms. Ingestion of toxins • Examples: Clostridium botulinum Staphylococcus aureus Exotoxins • Produced inside bacteria and secreted – Enterotoxin, Neurotoxin, Cardiotoxin • Superantigen: Type IÆintense immune responseÆcytokinesÆfever, nausea, vomitting, shock (Staph aureus) • Membrane disrupting toxin: Type IIÆ lysis of cells by disrupting membrane • A-B toxins: Type III has two parts – A is active enzyme toxin that inhibits protein synthesis and kills cell – B binds to surface so toxin is transported across plasma membrane (Clostridium) 3 Clostridium botulinum : botulism • General characteristics: gram positive rod, anaerobe, spore former • Produces a toxin (neurotoxin) – Heat sensitive – One gram can kill 1 million • Toxin inhibits the release of acetylcholine from neurons…..what happens next? Botulism • Foods associated: home canned “low acid” vegetables, honey • Symptoms:12-72 hours after ingestion vomiting, diarrhea, blurred vision, and descending muscle weakness • Treatment: antitoxin not antibiotics Staphylococcus aureus: 24 hour Flu? • General characteristics: gram positive coccus in clusters, facultative anaerobe, part of normal skin flora • Produces a toxin – Enterotoxin (targets the GI tract) – Acts as a Superantigen 4 Staphylococcus aureus • Foods associated: cream based desserts, custards, potato and egg salads – Key is to have a food handler with the organism as part of the normal skin flora – Remember aw coefficient for this organism is low (0.86) – Food left at 280C for 2-4 hours with S. aureus will have enough cells grown to cause food poisoning Staphylococcus aureus • Symptoms: appear 1-6 hours after ingestion and include vomiting (V), diarrhea (D), and intense abdominal pain/cramping(AB), usually no fever -last approximately 24 hours • Treatment: none, supportive care 5 Food poisoning • Food borne infection: bacteria enter food and grow within the food. Ingestion of organism (may then produce toxins that cause symptoms) • Examples: Campylobacter jejuni, Salmonella spp., Salmonella typhii, Shigella spp., Vibrio cholerae, Vibrio parahemolyticus, Vibrio vulnificus Campylobacter jejuni • General characteristics: gram negative curved rod, microaerophile, one or two polar flagella, no capsule • Special culture to grow: microaerophilic, special media, 42⁰C incubator • Part of the normal flora of poultry and dairy cattle -unpasteurized milk, undercooked poultry -may be found at a concentration of 109cells/gram of chicken Campylobacter jejuni • Incubation period: 2-5 days • Symptoms: D (which may be bloody), AB, fever (1040C), vomiting not common – Last 2-10 days – Some cases lead to Guillen-Barre syndrome and Rheumatoid Arthritis weeks after the illness 6 Campylobacter jejuni • Treatment: none, mostly supportive care -some cases require antibiotics (erythromycin, quinolones) • Avoid undercooked poultry and watch the use cutting boards in food prep Salmonella spp. • General characteristics: gram negative rod, facultative anaerobe, peritrichous flagella • Over 2000 closely related serovars (serotypes) • Part of the normal flora of poultry, reptiles • Culture: isolate and identify lactose negative, H2S positive Salmonella • Incubation time: 12-36 hours • Symptoms: include D, AB, and a moderate fever • Full recovery in a few days but may shed the organism for 6 months • Approx. 2-4 million cases/year, only 4050,000 are actually reported 7 Salmonella typhii • Causes typhoid fever (enteric fever) • Most Salmonella killed by acid so need to ingest large numbers to survive stomach acid in order to cause infection • Only found in the feces of other humans • Some individuals (Typhoid Mary) become carriers (1-3%), usually colonizes gallbladder (free of competition from NF due to bile) Salmonella typhii • Incubation time: 2 weeks • Symptoms: high fever (1040C), headache, chills for one week -transition in the second week to D, fever declines Only 300- 500 cases annually • Treatment: antibiotics 8 • Salmonella typhimurium and enteritidis: – Most common serovars in USA – Antibiotics not advised due to increasing resistance (due to widespread addition of antibiotics to animal feed) – Not treat gastroenteritis unless invades tissues Shigella spp. • General characteristics: gram negative rod facultative anaerobe, lactose negative • Only found in the feces of other humans • Organisms transmitted by the five F’s -food, fingers, feces, flies, and fomites Shigellosis or bacillary dysentery Shigella 9 Shigella • Incubation time: 24-48 hours – Only need 10 cells to cause infection; not affected by stomach acid • Symptoms: F, AB, D (may contain blood and mucus) • See passage of small volume bloody stools (20/day) Shigella • S. sonnei-most common species in US, responsible; relatively mild; may cause some of traveler’s diarrhea • S. dysenteriae-causes more serious infection dysentery -due to the production of a Shiga toxin (A-B toxin) -in tropical areas-death rate up to 20% Dysentery • Shigella dysenteriae – Virulent strains have plasmids to attach to epi cells of intestine; membrane ruffles around and engulfs bacteria which multiply in cell and produce Shiga toxin which destroys tissue; also has mechanism that allows to spread from cell to cell; sloughed areas of epis leads to intense inflammation, pus, and bleeding – Rarely invades blood – Toxin strongly associated with hemolytic uremic syndrone; RBCs break in tiny blood vessels leading to anemia and kidney damage 10 Vibrio cholerae • Curved gram negative rod, facultative anaerobe, single polar flagella • Can exist in saltwater for extended periods of time; tolerates high pH and high salt concentrations • Killed by stomach acid so need large numbers of organisms to cause infection Vibrio cholerae 11 V. cholerae • Incubation time: several hours to days • Symptoms: “rice water stools”, sudden onset of explosive watery diarrhea (up to 20 liters/day) with vomiting and pain • Cholera toxin is the key pathogenic feature – A-B toxin causes activation on enzyme that causes cells to continuously secrete chloride ions and other electrolytes and H2OÆ watery diarrhea Non-cholera Vibrios • V. parahemolyticus: halophilic (requires sodium); inhabits coastal salt waters; usually transmitted by raw or undercooked shellfish; less severe gastroenteritis • V. vulnificus: halophilic (requires 1% NaCl); wound infections—minor skin infection in coastal waterÆrapid sread through tissuesÆpossible amputation – Fatal specticemia in 50% of patients with liver disease 12 Escherichia coli • Normal bowel flora • Strains that cause GI disease have virulence factors (coded by plasmids) – Enterotoxin production – Abiltiy to adhere to small intestine Diarrhea causing E. coli • Classified according to virulence – Entertoxigenic E. coli (ETEC) – Enterpathogenic E. coli (EPEC) – Enterohemorrhagic E. coli (EHEC) – Enteroinvasive E. coli (EIEC) Entertoxigenic E. coli (ETEC) • Also known as traveler’s diarrhea • Two Enterotoxins promote the pumping of Cl- and inhibition of NaCl which results in diarrhea – Profuse watery stools • No invasion • Can develop immunity • Prevent with bismuth (Pepto-Bismol) 13 Enterpathogenic E. coli (EPEC) • Attacks the small intestine • In developing countries accounts for 20% of diarrhea in bottle-fed infants • Attaches to mucosa of small intestine and causes cell surface changes (loss of microvilli) Enteroinvasive E.Coli (EIEC) • Invades lining of large intestine causing necrosis, inflammation, and ulceration of large bowel • Usually seen in children inn areas with poor sanitation Enterohemorrhagic E. coli (EHEC) • • • • Obtain from the consumption of animal products Small dose (< 100 bacteria) to infect Attacks the colon Produces Shiga like toxin and lesion; inflammation and bleeding (hemorrhagic colitis) • O157:H7 causes bloody diarrhea which may lead to hemolytic uremic syndrome • Sorbitol negative MacConkeys used to isolate; culture all bloody diarrhea 14 Listeria monocytogenes • Gram positive rod, non-spore forming • Infects GI tract (usually few symptoms), penetrates mucosa and enters blood stream • Fourth leading cause of meningitis (perhaps 50% fatality rate) • Pregnant women: crosses placenta and causes absess in fetal tissueÆmiscarriage or stillbirth (60 %) • Can grow at 4⁰C (refrigerator) • Outbreaks from coleslaw, unpasteurized milk, soft cheeses, hot dogs Bacillus cereus • Spores are ubiquious in nature; can cause food poisoning; diarrhea and vomitting • Most commonly opportunistic (post trauma to eye) • Heating does not kill spore but will kill other competing bacteria • Rice in Asian restaurant Yersinia enterocolitica • Gram negative rod; lactose negative; grows at 4⁰C • Inhabits domestic animals (contaminates meat and milk) • Fever, diarrhea, abdominal pain • Can invade mucosa and spread to lymphatics (may present as appendicitis) • Implicated in contamination of transfused blood 15 16